USOO5792235A United States Patent (19) 11 Patent Number: 5,792,235 Kohr 45) Date of Patent: *Aug. 11, 1998

54 METHOD FOR RECOVERNG GOLD AND 5,443,621 8/1995 Kohr ...... 75/711 OTHER PRECOUS METALS FROM FOREIGN PATENT DOCUMENTS CARBONACEOUS ORES 1062918 9/1979 Canada. 75) Inventor: William J. Kohr, San Mateo, Calif. 0229224 9/1986 European Pat. Off. . 1105237 1/1983 Japan. 73) Assignee: Geobiotics, Inc., Hayward, Calif. 2136015 9/1984 United Kingdom. OTHER PUBLICATIONS * Notice: The term of this patent shall not extend beyond the expiration date of Pat. No. V. V. Lodeischikov, "Recovery of Gold from Refractory 5,443,621. Ores and Concentrates", pp. 177-185 (Chapter XI). Pub lished: NEDRA. Moscow, 1968 (along with the partial 21 Appl. No.: 415,745 English translation) month unavailable. Schulze, R.G., Thiourea Leaching Of Precious Metals, 96 22) Fed: Apr. 3, 1995 Erzmetall, 39 (1986) Feb., No. 2, Weinheim, Deutschland. Scheiner, B.J., et al., Processing Refractory Carbonaceous Related U.S. Application Data Ores for Gold Recovery. Journal of Metals. pp. 37-40. Mar. 1971, 63) Continuation of Ser. No. 24,331, Mar. 14, 1994, aban doned, which is a continuation of Ser. No. 950,576, Sep. 22, Guay, W.J. The Treatment of Refractory Gold Ores Con 1992, Pat. No. 5,338,338. taining Carbonaceous Material and Sulfides, Society of Mining Engineers of AIME, Reprint No. 81-45, pp. 1-4 51) Int. Cl...... C22B 3/06; C22B 3/16 Feb. 22-26, 1981. (52) U.S. Cl...... 75/711; 423/22; 423/26; Hutchins, S.R., et al. Microbial Pretreatment of Refractory 423/27; 423/DIG. 17 Sulfide and Carbonaceous Ores Improves the Economics of (58) Field of Search ...... 75/711, 732; 423/22. Gold Recovery . Mining Engineering, pp. 249-254 (Apr. 423/26, 27. DG. 17 1988). Scheiner, B.J. et al., Oxidation Process for improving Gold (56) References Cited Recovery from Carbon-Bearing Gold Ores, U.S. Dept. of U.S. PATENT DOCUMENTS the Interior, Report of Investigations 7573, 1971. Han, K., et al. Separation of Cabonaceous Material from 2,838,369 6/1958 Gaudin, et al...... 23/14.5 Carlin Ore by Flotation, Advances in Gold and Silver 2,890,795 6/1959 Dering ...... 209/12 Processing. Conference Proceedings, Reno, Nevada, Chap 3,450,523 6/1969 Socolescu ...... 75/7 3,574,600 4/1971 Scheiner, et al...... , 75/105 ter 13, pp. 121-130 Sep. 10-12, 1990. 3,635,697 1/1972 Scheiner, et al. . ... 75/101 Afenya, P.M., Treatment of Carbonaceous Refractory Gold 3,639,925 2/1972 Scheiner, et al. ... 75/101 Ores, Mineral Engineering, vol. 4, pp. 1043-1055 (1991). 3,703,366 11/1972 Cullom ...... 75/74 Radtke, A. Studies of Hydrothermal Gold Deposition (I). 3,764,650 10/1973 Scheiner, et al...... 423/38 Carlin Gold Deposit, Nevada: The Role of Carbonaceous 3,979,205 9/1976 Wanzenberg...... 75/10 Materials in Gold Deposition. Economic Geology and the 4,188,208 2/1980 Guay ...... 75/105 Bulletin of the Society of Economic Geologists, pp. 87-102, 4,289,532 9/1981. Matson, et al...... , 75/105 vol. 65, Mar.-Apr. 1970, No. 2. 4,347,126 8/1982 McGarry, et al. . ... 209/164 Scheiner, B.J. Relation of Mineralogy to Treatment Methods 4,369,061 1/1983 Kerley ...... 75/732 for Carbonaceous Gold Ores, Society of Mining Engineers, 4,552,589 11/1985 Mason, et al. ... 75/105 4,554,137 11/1985 Maurice ...... 423/26 pp. 1-6, Preprint No. 87-96 Feb. 24-27, 1987. 4,556,500 12/1985 Bresson, et al...... 252/61 Johns, M.W., et al., Recovery of Gold From Ashed Wood 4,578,163 3/1986 Kunter, et al...... 204/110 chips, J.S. Afr. Inst. Min. Metall., vol. 90, No. 1. pp. 1-10 4,585,550 4/1986 Avotins, et al. ... 209/166 (Jan. 1990). 4,610,724 9/1986 Weir, et al...... 7.5/118 4,702,824 10/1987 Abadi ...... 209/167 (List continued on next page.) 4,710,361 12/1987 Ostrea ...... 423/26 Primary Examiner-Melvyn Andrews 4,786,323 11/1988 Gock, et al. ... 75/118 Attorney, Agent, or Firm-Lyon & Lyon LLP 4,814,007 3/1989 Lin, et al...... 75/118 4,895,597 1/1990 Lin, et al. ... 75/118 57 ABSTRACT 4,900,431 2/1990 Cariou, et al. .. 209/166 4,902,345 2/1990 Ball, et al...... 75/18 Amethod for recovering precious metals from carbonaceous 4,923,510 5/1990 Ramadorai, et al. ... 423/29 ore comprising leaching the ore with a lixiviant solution and 4,950,390 8/1990 Szentiaszloi, et al. 209/64 then preg-robbingly concentrating the precious metal 5,013,359 5/1991 Fair, et al...... 75/744 lixiviant complexes in solution on to the native carbon 5,021,088 6/1991 Portier ...... 75/736 aceous component of the ore for subsequent recovery. The 5,051,199 9/1991 Barwise ...... 252/61 5,073,354 12/1991 Fuller, et al. ... 423/24 preg-robbing capacity of the native carbonaceous compo 5,074,909 12/1991. Agar...... 75/422 nent of the ore can be augmented by adding recycled

5,147,618 9/1992 Touro ...... 423/27 carbonaceous matter or finely ground carbon to the ore 5,320,720 6/1994 Hayden, et al. 204/110 lixiviant mixture. 5,338,338 8/1994 Kohr ...... 75/711 5,364,453 11/1994 Kohr ... 75/711 18 Claims, No Drawings 5,792,235 Page 2

OTHER PUBLICATIONS Williams, M.C.. et al., A Simple Flotation Method for Rapidly Assessing the Hydrophobicity of Coal Particles, Han, K.N., et al. Using Flotation to Separate Carbon International Journal of Mineral Processing. 20 (1987) pp. Material from Carlin Ore, Mining Engineering, vol. 42, No. 153-157. 12, pp. 1328 (Dec. 1990). Ibrado, A.S., et al., Effect of the Structure of Carbon Adsor Lichy, L., et al., Treatment Refractory Ores: A Simplified bents on the Adsorption of Gold Cyanide, Hydrometallurgy, Process, Mining Engineering, vol. 42, No. 12, p. 1328 (Dec. 30 (1992) 243-256, Elsevier Science Publishers B.V., 1990). Amsterdam. Crabtree. E.H., Jr., et al. Developments in the Application of Fuerstenau, D.W., Characterization of Coal Particle Sur Activated Carbon to Cyanidation (Including the Desorption faces by Film Flotation. Proccedings of Surface Chemistry of Gold and Silver from Carbons). Mining Eng., Transac of Coal, Los Angeles, Ca, Sep. 25-30, 1988, pp. 748-755. tions AIME. vol. 1987, pp. 217-222 (Feb. 1950). Orlich, J.N. Column Flotation of Carbon at the Royal Mt. Rosenbaum, J.M., et al., Benefication of Fine Western Coal King Mine, SME Annual Meeting-Phoenix, Arizona, Feb. by Froth Flotation. The American Institute of Chemical 24-27, 1992. Engineers pp. 19-28 (1982). Fuersteanau. et al. Coal Surface Control for Advanced Fine Scheiner. B.J., Carbonaceous Gold Ores. Inf. Circ., U.S. Coal Flotation. Project No. DE-AC22-88PC88878, Quar Bureau of Mines, pp. 26-33 (1986). terly Report No. 11, Apr. 1 through Jun. 30, 1991. 5,792.235 1. 2 METHOD FOR RECOVERNG GOLD AND with these ores, however, is indirect interference. This OTHER PRECOUS METALS FROM occurs when the gold-lixiviant complex formed during lix CARBONACEOUS ORES iviation is sorbed by the native carbonaceous material and, therefore, is no longer available for recovery from solution. This is a continuation of application Ser. No. 08/214.331 5 This phenomenon is called preg-robbing. filed on Mar. 14, 1994 now abandoned, which is a continu Preg-robbing is frequently associated with the use of ation of application Ser. No. 07/950,576 filed Sep. 22, 1992. cyanide as the lixiviant. However, it also occurs with gold now U.S. Pat. No. 5,338.338. lixiviant complexes other than aurocyanide. Certain clay materials such as illite, kaolin, and montmo BACKGROUND OF THE INVENTION 10 rillonite are also known to preg-robbingly adsorb the gold cyanide complex. Thus, the degree of preg-robbing exhib 1. Field of the Invention ited by an ore depends on the amount of carbonaceous The present invention relates to the recovery of precious matter and preg-robbing clay materials in the ore. As used metals from carbonaceous ores. More particularly, the herein, it should be understood that carbonaceous compo invention concerns an improved leach of these ores wherein 15 nent and carbonaceous matter also refer to preg-robbing the native preg-robbing component is used to concentrate clays, because the preg-robbing properties of these materials gold or other precious metals for subsequent recovery. are functionally similar to that of the actual carbonaceous 2. Description of the Prior Art matter in the ore. Gold is one of the rarest metals on earth. Gold ores can be While preg-robbing is most frequently associated with categorized into two types: free milling and refractory. Free 20 cyanidation processes, the preg-robbing phenomenon is also milling ores are those that can be processed by simple known to occur with other gold-lixiviant complexes such as gravity techniques or direct cyanidation. Refractory ores, on gold-chloride. The inventor has even experienced preg the other hand, are difficult to process. Refractory ore robbing of gold-thiourea complexes while using a thiourea resources can consist of ores, flotation concentrates, mill lixiviant. tailings, and other reserves. In the past, refractory ores have 25 Carbonaceous ores vary significantly from deposit to required pre-cyanidation treatments to liberate the gold. The deposit, and even within deposits, in the amount of carbon difficulty of processing refractory gold ores is attributable to aceous matter they contain. These ores have been reported to their mineralogy. contain from approximately 0.2% carbon to as much as 5% A large number of refractory ores consist of ores with a carbon. P. Afenya, Treatment of Carbonaceous Refractory precious metal such as gold occluded in iron sulfide par 30 Gold Ores, Minerals Engineering. Vol. 4, pp. 1043-1055, ticles. The iron sulfide particles consist principally of pyrite 1991. and arsenopyrite. If the gold remains occluded, even after If P represents the preg-robbing component of the ore, V fine milling of these ores, then the sulfides must be oxidized represents a valuable mineral component (i.e., gold, silver, to liberate the encapsulated precious metal and make it or platinum), and G represents the gangue materials in the amenable to a leaching agent (or lixiviant). 35 ore, then preg-robbing may be illustrated by the following Carbonaceous gold ores represent a unique class of refrac general formula: tory ores. Not only is gold sometimes found encapsulated in sulfide minerals in these ores, but these ores also contain carbonaceous matter that interferes with recovery by cyani Wherein V represents the precious metal closely associated dation. Gold in carbonaceous ores, therefore, can be asso with the preg-robbing material in the ores, V represents the ciated with sulfide minerals, carbonaceous matter, and/or precious metal associated with gangue material, V repre siliceous minerals. P. Afenya, Treatment of Carbonaceous sents the precious metal preg-robbingly removed from the Refractory Gold Ores, Minerals Engineering, Vol. 4, Nos lixiviant solution, V represents the precious metal-lixiviant 7-11, pp 1043-55, 1991, hereby incorporated by reference. 45 complexes remaining in solution, and V-represents the The distribution of gold in these mineral groups can vary amount of precious metal remaining associated with the considerably from ore to ore. gangue material after lixiviation. Researchers have identified the carbonaceous matter in Thus, the amount of precious metal that is associated with these ores as containing (1) an activated carbon component the preg-robbing component of the ore after lixiviation is capable of adsorbing gold-chloride complexes and gold 50 equal to the amount of precious metal originally associated cyanide complexes from solution, (2) a mixture of high with the preg-robbing component of the ore plus the amount molecular weight hydrocarbons usually associated with the that is preg-robbingly removed from the lixiviant solution activated carbon components; and (3) an organic acid, (V). The amount of precious metal remaining associated similar to humic acid containing functional groups capable with the gangue material (V) is equal to the original of interacting with gold complexes to form organic gold 55 amount of precious metal (V) minus the amount of precious compounds. P. Afenya, Treatment of Carbonaceous Refrac metal dissolved by the lixiviant (V-V). tory Gold Ores, Minerals Engineering, Vol. 4, pp. A number of techniques have been developed for pro 1043-1055, 1991. hereby incorporated by reference; W. cessing refractory carbonaceous gold ores. These techniques Guay. The Treatment of Refractory Gold Ores Containing include flotation, blanking, carbon in leach, roasting, chemi Carbonaceous Material and Sulfides, Society of Mining cal oxidation, and bacterial leaching. Roasting and oxidation Engineers of AIME, 81-34, pp. 1-4, 1981, hereby incorpo by chlorination are the two methods that are most developed rated by reference. and applicable for treating carbon-bearing ores. The others Carbonaceous matter, can therefore directly or indirectly may play some role in the future or are often confused with interfere with lixiviation. Directinterference with lixiviation methods for processing carbonaceous ores, even within the is ascribed to either occlusion of the gold within the car 65 mining industry, when they are really more suited to treating bonaceous material or formation of a stable gold-carbon refractory sulfidic ores. The various techniques are complex similar to a chelate. The more common problem described below: 5,792,235 3 4 1. Flotation and Depression the principle of using a stronger aurocyanide adsorbent than This method has been employed successfully where small the carbonaceous matter in the ore. P. Afenya, Treatment of amounts of gold are associated with the carbonaceous matter Carbonaceous Refractory Gold Ores, Minerals Engineering, in the ore. In such circumstances, the carbonaceous matter Vol. 4. pp. 1043-1055, 1991. However, this process is not can be floated off and discarded. The remaining ore is then effective when the ore contains large amounts of carbon processed using conventional cyanidation techniques. This aceous matter, because native carbonaceous matter has the technique, however, does not work for ores in which con ability to adsorb gold cyanide complex four times faster than siderable quantities of gold are associated with the carbon aceous component. J. Orlich, J. Fuestenau, & D. Horne, activated carbon. B. J. Scheiner, Relation of Mineralogy to Column Flotation of Carbon at the Royal Mt. King Mine, Treatment Methods for Carbonaceous Gold Ores, Society of SME Annual Meeting. Phoenix, Ariz., February 1992. Mining Engineers, 87-96, pp 1-6, 1987. Furthermore, CIL One mining operation has tried to produce a high grade processes use relatively large carbon particles, whereas the concentrate for possible shipment to a smelter and a tailing ore is fine ground, so that the added carbon and its adsorbed which could be discarded or directly cyanided. W. Guay, The gold values may readily be separated from the ore after Treatment of Refractory Gold Ores Containing Carbon cyanidation by size. aceous Material and Sulfides, Society of Mining Engineers 15 4. Roasting of AIME. 81-34, pp. 1-4, 1981. The concentrates contained This is the current industry standard for simultaneously both carbonaceous materials and pyrite, but exhibited low destroying carbonaceous matter, and simultaneously oxidiz recoveries of gold. ing the sulfide minerals, in refractory carbonaceous gold According to the process disclosed in U.S. Pat. No. ores. In fact, the majority of recently built pretreatment 4.585.550, hereby incorporated by reference, a coal fraction 2 plants use roasting. In Nevada, four roasters have been put containing economically significant concentrations of a into operation since 1986, and at least one more is in the desirous mineral value can be recovered from a carbon planning stage. aceous ore by flotation. However, under this process, gold Modern roasters use a fluidized bed construction and values contained in the non-floated fractions of the ore are conventional fuel source to heat the ores. Roasting tempera lost; thus, this process can only be used if small amounts of 25 tures are usually between 600 and 700° C. After roasting, gold are associated with the unrecovered fractions. the ore is separated from dust and off-gasses and then Other goldfields have depressed the carbonaceous com quenched. Following quenching. the oxidized ore can be ponent of the ore while floating the sulphide minerals and processed using traditional cyanide extraction techniques. free gold. P. Afenya, Treatment of Carbonaceous Refractory For any particular ore composition, roasting plants oper Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055. 30 ate in a narrow range of tolerances. Below optimum tem 1991. Again, however, this technique would not be used if perature the carbon in the ore is not oxidized and remains the carbonaceous component contained significant quanti actively preg-robbing. Above the optimum temperature, the ties of gold. gold in the ore becomes increasingly less amenable to A common problem with all of the flotation processes, cyanidation or other extraction techniques. Because of the therefore, is that the gold associated with the ore fraction 35 degrading gold recovery with higher temperatures, many that is to be discarded is lost because it is generally uneco roasters are operated toward the lower side of the range. nomical to recover. As a result, the tail fraction must contain Blanking agents are then added to passivate any unroasted very small amounts of gold for the existing flotation pro carbonaceous matter. Accordingly, roaster efficiency in a cesses to work satisfactorily. However, the mineralogy of a plant environment tends to vary widely with variation in carbonaceous gold ore deposit is continually changing. feed stock. Therefore, as the amount of gold associated with the ore For many years roasting was the only reliable method of fraction that is to be discarded (i.e., the tail) increases, the treating refractory carbonaceous gold ores to produce high amount of gold values lost during flotation also increases. gold recovery. In the last two decades, however, the increas Current flotation processes are not flexible enough to com ing costs associated with roasting has increased the pressure pensate for these changes in the mineralogies of carbon 45 to find alternative methods for treating refractory carbon aceous gold ores. The present invention overcomes this aceous gold ores. Roasting costs are driven in large part by problem by preg-robbingly concentrating the gold values in two factors: energy economics and environmental regula the carbonaceous component of the ore prior to flotation. tion. Energy sources are used for both heating and process 2. Blanking control, such as oxygen injection. As a result, this method is Blanking agents are used to passivate the surfaces of particularly sensitive to fluctuations in fuel prices. Environ activated carbon in carbonaceous ores. The blanking agents mental regulation is also a large and growing cost factor in work by selectively adsorbing on the surface of the activated the operation of roasters. The off-gas must be treated to carbon preferentially to the gold-lixiviant complexes in suppress dust and to remove extremely toxic mercury and solution. Kerosene, fuel oil, and RV-2 (para nitrobenzolazo arsenic compounds and sulfur dioxide. This is often accom salicylic acid) have been used as blanking agents. This 55 plished using electrostatic precipitators and scrubbers. These method is not applicable where considerable quantities of pollution control technologies, however, are both expensive gold are associated with the carbonaceous matter. And as and difficult to control. explained in U.S. Pat No. 3,574,600, blanking is also not As emission standards become stricter, roasting process applicable to ores that contain significant quantities of costs increase dramatically. Almost without exception, both organic acids as carbonaceous matter. One of the objects of analytical studies and actual operators estimate the cost of the present invention is to permit the processing of carbon roasting to be in the area of $10 to $20 per ton of ore, aceous ores regardless of native carbon content and regard although one source claims an estimate for a proposed plant less of the amount of gold originally associated with the of $8 per ton. carbonaceous matter. 5. Chemical Oxidation 3. Activated Carbon or Resin In Leach or Pulp 65 Currently, hydrometallurgical methods for treating refrac Activated carbon or resin can be added to leach solutions tory gold ores strongly attract research and development to preferentially adsorb aurocyanide. This process rests on activity. Currently, there are three aqueous oxidation tech 5,792.235 5 6 niques being given attention: (1) chlorine oxidation, (2) that a traditional cyanide process could be used to recover autoclave leaching and (3) bioleaching. Bioleaching is dis the precious metal values from the ore. The inventor's cussed separately. process is a completely novel approach in which the here a. Chlorination tofore deleterious preg-robbing characteristic of carbon This was the method most favored until process econom aceous ores is used advantageously to concentrate the pre ics and environmental regulation tipped the scale in favor of cious metal values in the carbonaceous ore on the preg roasting. At least two chlorination plants were operating robbing component of the ore for subsequent recovery. recently, although one of them may already be off line. At present, there are large amounts of both located In this process, the ore is ground and mixed with water to carbonaceous deposits and stocks of mined carbonaceous form a slurry, Chlorine gas is pumped into the sluny under 10 ore that have been set aside because they cannot be pro pressure at a rate of about 60 to 120 lbs/ton, depending on cessed economically using current methods. the residence time, organic carbon concentration in the ore, SUMMARY OF INVENTION and percent solids in the slurry. The chlorine gas will oxidize the carbon in the ore. rendering it less preg-robbing. After The present invention is directed to methods for recov treatment, the hypochlorous acid generated must be treated 15 ering gold from carbonaceous gold ores whereby the car with a reducing agent to prevent it from destroying the bonaceous component contained in the carbonaceous ores is cyanide used later in the process. used to concentrate the gold for subsequent recovery. To this This process is particularly sensitive to the amount of end, carbonaceous ore is contacted with a lixiviant solution sulfide in the ore, because sulfur is oxidized before carbon. thereby causing the production of gold-lixiviant complexes Higher sulfide ores require much more chlorine gas. For 20 and the dissolution of gold from the ore. The carbonaceous very refractory ores the "Double Oxidation" process component of the ore preg-robbingly concentrates the gold described in P. Afenya, Treatment of Carbonaceous Refrac lixiviant complexes in solution and is then separated from tory Gold Ores, Minerals Engineering, Vol. 4, pp. the bulk of the gangue material to form a concentrate. In a 1043-1055, 1991, hereby incorporated by reference, has preferred embodiment of the present invention, gold is been used. 25 recovered from the carbonaceous component. The process is Environmental factors also play a large part in driving also applicable for recovering other precious metals such as costs. Gas emissions from the tanks must be captured by silver and platinum from carbonaceous ores containing the alkaline scrubbers before being released to remove the S2. chlorine they contain. High pressure chlorine gas is Accordingly, it is an object of the present invention to extremely dangerous. 30 provide an economical and effective process for recovering Finally, the process is difficult to control in operation, and gold and other precious metals from carbonaceous ores plants suffer from the corrosive gas. As a result of all of these using the inherent preg-robbing capabilities of these ores to factors, roasting will be the economically favored alternative concentrate gold and other precious metals in the carbon to chlorine based oxidation for the foreseeable future. aceous component of the ore prior to separation. Additional As a variant of chlorination, NaOCl can be substituted for 35 and further objects and advantages of the present invention chlorine gas as the oxidizing agent. Furthermore, NaOCl can will appear hereinafter, be produced in situ by electrolyzing NaCl. The NaOCl is DETALED DESCRIPTION OF THE used in the same manner as the chlorine above to oxidize PREFERRED EMBODIMENTS sulfides and carbonaceous matter in the ore. However, the initial capital investment for this technique is high, and The starting materials upon which the present invention unless there is a radical decrease in energy costs, this method operates have been termed "carbonaceous ores,” a specific will remain even less economically attractive than chlori type of gold ore that contains a carbonaceous component nation. capable of adsorbing various gold-lixiviant complexes b. Pressure Autoclaving including Au(CN), AU(SCNH) and AuCl. While any This method is far more successful at oxidizing sulfidic 45 lixiviant that forms a gold-lixiviant complex that is adsorbed materials that make the ore refractory than it is at oxidizing by the carbonaceous component of the ore may be used in carbonaceous matter that may be present. It is mentioned the present method, cyanide is the preferred lixiviant. here for the sake of completeness. A pressure autoclaving According to the present method, carbonaceous ore is process followed by CIL is taught in U.S. Pat. No. 4,552, leached with a lixiviant solution to dissolve the gold from 589, hereby incorporated by reference. 50 the bulk of gangue material. Thereafter, the gold-lixiviant 6. Bioleaching complexes formed by leaching are preg-robbingly removed This is the latest process being developed to treat refrac or sorbed by the carbonaceous component of the ore. After tory sulfide and carbonaceous gold ores. The process uses the gold is concentrated on the carbonaceous component of bacteria to biologically degrade sulfide minerals and liberate the ore, the carbonaceous component is separated from the precious metal values so that they can be recovered by 55 ore and the gold recovered. Any of the conventional sepa conventional technologies. The most widely used and stud ration techniques known in the art may be used, including ied bacteria for this process is Thiobacillus ferrooddans. gravitational and froth flotation. The preferred separation Bioleaching, however, has little effect on the preg-robbing technique is froth flotation, with column flotation being a characteristics of an ore. Therefore, carbon-in-leach or preferred method of froth flotation. blanking has been used in addition to bioleaching to obtain Carbonaceous ores that can preg-robbingly remove about satisfactory gold yields from carbonaceous ores. 10 pig Au/g ore or more in 16 hours or less from a cyanide Furthermore, it takes days rather than hours to treat the ore. solution spiked with 4 ppm Au are preferred in practicing the Thus, since the mining of low grade carbonaceous gold present invention. Carbonaceous ores that can preg ore began more than 40 years ago, the mining industry has robbingly remove about 140 IgAu/gore or more in 16 hours repeatedly tried to find alternative methods of treating 65 or less are most preferred. The ore should be finely ground carbonaceous ore. These methods have all involved trying to to a particle size of at least -200 mesh prior to being eliminate or block the preg-robbing effect of these ores so contacted with the spiked gold cyanide solution. 5,792,235 7 8 The process in a particularly preferred embodiment of the mills, and the like. Grinding technologies that produce a present invention comprises: narrow particle size distribution are preferred. Ball milling (a) contacting ground carbonaceous ore with a lixiviant will produce a wide distribution of particle sizes. Hydrocy solution to form a slurry and thereby cause the produc clones can be used to separate larger particles from the ball tion of gold-lixiviant complexes, which result in the mill output for the purpose of regrinding before cyanidation dissolution of gold from the ore; and froth flotation. After the first flotation, hydrocyclones (b) preg-robbingly removing the gold-lixiviant complexes may also be used to remove larger particles of ore that still from solution to the carbonaceous component of the contain carbonaceous material that was not ground free of Ore: the gangue. These larger particles may contain adsorbed 10 gold and can be reground, recyanidated and refloated in (c) conditioning the slurry with a collector; order to recover more gold. (d) adding a frother to the conditioned slurry; Wet or dry grinding may be used to reach the final particle (e) separating the gold containing preg-robbing carbon size. However, if a wet grinding process is used, it is aceous component from the bulk of gangue material by preferred that the grinding be carried out in the presence of froth flotation; and 15 a lixiviant. (f) recovering gold from the carbonaceous component. Once the carbonaceous gold ore is ground, it is leached In this embodiment of the invention, the carbonaceous with a lixiviant. Lixiviant as used herein is a solvent that is component of the ore is to be separated from the bulk of used to dissolve the gold in the carbonaceous gold ore by gangue material by froth flotation. The preferred method of forming soluble gold-lixiviant complexes. Cyanide is the froth flotation being column flotation. The largest particle 20 preferred lixiviant for practicing the present invention. present within a mass of mineral particles, which are to be However, other lixiviants such as aqua regia, thiourea, separated by froth flotation, must be of a size such that the halide ion lixiviants and the like may also be used. desired mineral particles will be physically released from Sufficient lixiviant should be added to dilute the solids unwanted mineral particles (or the gangue) and that the mass concentration of the ground ore to the range of between of each of the desired mineral particles does not exceed its 25 100-600 gm/Kg, preferably about 400 gm/Kg. Naturally, if force of attraction to an air bubble under the conditions of the ore is wet ground, less lixiviant solution, if any at all, will turbulence occurring in the aqueous suspension of mineral need to be added to dilute the solids concentration into the particles. Because the carbonaceous component of the ore is above range. The appropriate concentration of lixiviant in to be floated in this preferred embodiment of the invention, the leach solution depends on the lixiviant being used to it is necessary to grind the carbonaceous gold ore fine 30 solubilize the gold in the ore and the desired leach rate. The enough so that the carbonaceous component is liberated typical concentrations of the various lixiviants used to leach from the gangue of the ore and the resulting particles of gold, however, are well-known in the leaching art. carbonaceous material are sufficiently small for separation The gold-lixiviant complexes formed during lixiviation by an industrial froth flotation process. In general, a final are adsorbed by the carbonaceous component in the carbon particle size of less than about 200 mesh is adequate. 35 aceous gold ore. This property of the carbonaceous compo However, as explained in U.S. Pat. No. 5,051,199, hereby nent of the ore has been called "preg-robbing." Preg-robbing incorporated by reference, overgrinding of the ore must be is believed to occur by both physical and chemical means. avoided because small carbonaceous particles that are very In the present invention, the process of preg-robbing, which small (less than approximately 1 micron) will not float as has traditionally been considered deleterious to the process well as larger particles. ing of these ores, is used to concentrate the gold-lixiviant Although -28 mesh is generally considered suitable size complex onto the carbonaceous component of the ore for for flotation, the nature of the ore being ground may require subsequent recovery. grinding to smaller sizes, e.g., -200 mesh, preferably -400 Lixiviation is carried out until equilibrium has been mesh, because flotation separation requires that the carbon reached or substantially reached between the gold in solu aceous matter and gangue or matrix materials be present as 45 tion and the gold adsorbed on the particles of carbonaceous distinct particles, separated from one another. matter in the ore. The time within which equilibrium is "Oxidized coal" particles are coal particles that are hydro achieved varies with such factors as particle size, philic and poor floating. These coal particles are hydrophilic temperature, concentration of lixiviant, and rate of agitation and poor floating because they are characterized by a high or stirring. oxygen content (i.e., many oxygen-containing functional 50 Naturally, the gold that is occluded in the activated carbon groups) at least on the surface of the coal particles. Thus, component of the carbonaceous matter or chemically bound even if the carbonaceous particles are already liberated in as a chelate by the organic acid component of the carbon the charge ore, the external surfaces of the coal particles will aceous matter is also recovered when the carbonaceous be the most oxidized areas, thereby making the carbon matter is subsequently separated from the bulk of gangue aceous particles difficult to float. And although the interior 55 material. of the carbonaceous particles may also be quite oxidized, After lixiviation, the slurry may be transferred from the they are generally less oxidized than the external surface. leaching vessel to a thickener and the slurry thickened for Consequently, grinding the carbonaceous particles to size subsequent flotation. Solids levels of over 100 gm/Kg, can have a profound effect on the overall effectiveness of the preferably over about 300 gm/Kg of the ore may be used in instant process. the flotation process. However, these levels are not critical Therefore, besides reducing the size of the ore to a size and higher or lower levels may oftentimes be used. small enough for flotation, i.e., smaller than about 28 mesh, If the carbonaceous gold ore does not contain sufficient and liberating the carbonaceous matter from the other matrix preg-robbing carbonaceous matter to completely adsorb the materials (generally silica, clays, and other silicates), grind gold-lixiviant complexes, then the decanted lixiviant ing also exposes fresh surfaces of the carbonaceous matter. 65 obtained from the thickener may be further processed by Grinding may be accomplished by any method known for running it through a column of activated carbon to adsorb mineral processing such as rod mills, ball mills, attrition the remaining gold-lixiviant complexes in solution. 5,792,235 10 In another preferred embodiment of the present invention the pH of the slurry is adjusted to about 9.0-12.0. Of course, recycled carbonaceous matter from processed ore or finely if the natural pH of the slurry falls within this range, no ground carbon may be added to augment the preg-robbing adjustment is necessary. capacity of the native carbonaceous matter. Such materials When the collector is added to the slurry, mixing for about as coal, activated charcoal. ashed woodchips, synthetic 0.1 to 30 minutes, preferably from about 1 to 10 minutes, is resins, and the like may be used as the finely ground carbon. conducted in order to ensure contact between droplets of the If finely ground carbon is used to augment the preg collector and the gold containing carbonaceous particles to robbing capacity of the native carbonaceous matter in the be floated. The conditioning time depends on many variables ore, it is preferred that the finely ground carbon and the including the collector composition and concentration, the native carbonaceous matter be of similar particle sizes. 10 degree of oxidation of the carbonaceous matter in the ore, An advantage of augmenting the natural preg-robbing and the solids concentration. Conditioning may be accom capacity of the ore is that the concentration of gold-lixiviant plished in the flotation cell or in a separate mixing vessel. complexes in solution is lowered, driving the dissolution The conditioner may also be added to the ore while it is reaction forward and improving the adsorption kinetics; being ground to size. thus, more gold is solubilized and then concentrated in the 5 A frother is added to the aqueous slurry, and then the carbonaceous component of the ore. Furthermore, augmen carbonaceous ore is floated in an appropriate flotation cell. tation ensures that sufficient carbon is floated off of the ore Prior to flotation, however, the slurry is again conditioned and that the overall removal of gold from the ore pulp is of for about 0.1 to 30 minutes, preferably from about 1 to 10 sufficient efficiency to be economically useful. minutes. The frothing agent permits a froth of the required In another preferred embodiment of the present invention, 20 stability to be produced during the subsequent flotation of after lixiviation, NaCl, (NH)SO, or NaSO salt is added to the aqueous slurry. Dowfroth 250 (polypropylene glycol the ore lixiviant slurry. Preferably (NH4)2SO, or NaSO is methyl ether), MIBC or Aerofroth 88 are the preferred added because NaCl may cause excessive corrosion of frothing agents. Dowfroth 250 is available from Dow processing equipment. The preferred salt concentration is Chemical in Midland, Mich... and Aerofroth 88 may be about 5 weight%. Salt additions increase the polarity of the 25 purchased from the American Cyanamid Co., Bountiful, water in the lixiviant. Thus, salt makes the hydrophobic Utah. carbonaceous component of the ore even less attracted to the During flotation of the aqueous slurry, a froth of the gold water and more attracted to the air in the flotation cell. containing carbonaceous matter is produced. The froth is If a cyanide solution is used as the lixiviant it may be skimmed off, thereby separating the gold containing carbon removed after cyanidation of the ore. The ore can then be aceous matter from the bulk of the gangue material. All resuspended in a 0.1N NaOH solution with 5% of (NH) non-floating particles are transferred to a thickener where a SO or NaSO salt added. The final pulp density of the flocculent can be added and the lixiviant can be recovered slurry is adjusted so that the solids level is over 100 gm/Kg, for reuse. Prior to reuse, if the ore contains an insufficient preferably over about 300 gm/Kg as indicated above. amount of carbonaceous material to adsorb substantially all Replacement of the cyanide solution with a 0.1N NaOH 35 of the gold-lixiviant complex in solution and its preg solution minimizes the potential for the formation residual robbing capacity is not augmented with recycled carbon hydrogen cyanide gas during the subsequent flotation step. aceous matter or finely ground carbon, the lixiviant may be Ca(OH) may be substituted for NaOH in the above solu stripped of any gold values by running it through an acti tions. wated carbon column. Before flotation, the aqueous slurry can be conditioned In order to provide a cleaner concentrate, the flotation with a collector. The collector is a chemical compound that concentrate from a rougher flotation cell may be floated a enhances the hydrophobic nature of the surface of the second time in a cleaner flotation cell. The concentrate from carbonaceous particles so that these particles are attracted to the cleaner flotation cell being the final concentrate of gold air rather than water. containing carbonaceous matter. The collector, which is used to render the carbonaceous 45 Gold can be recovered from the concentrate of gold component of the ore hydrophobic, may be any of the containing carbonaceous matter by either ashing the carbon collectors conventionally used in the benificiation of car aceous matter in a roaster or stripping the gold using an bonaceous matter by froth flotation. Some of the conven eluant such as hot cyanide. Such a stripping process is tional collectors that may be used include: motor oil, high disclosed in U.S. Pat No. 4,188,208, hereby incorporated by purity vacuum pump oil, kerosene, paint thinner, fuel oil. SO reference. The process disclosed in U.S. Pat. No. 3,979.205, plant oils and the like. Aromatic oils such as those described hereby incorporated by reference. may also be used to in K. Han, et al., Separation of Carbonaceous Material from recover the gold from the floated carbonaceous component Carlin Ore by Flotation, Conference proceedings: Advances of the ore. in Gold and Silver Processing. Reno, Nev., Sep. 10-12, In another aspect of the present invention, the process 1990, Society for Mining, Metallurgy, and Exploration, Inc., 55 further comprises a technique for processing carbonaceous p. 121, hereby incorporated by reference, may also be used. gold ores that contain occluded gold in the sulfidic mineral The preferred collectors of the present process are jojoba oil component. If economically significant quantities of gold and meadowfoam oil. remain occluded in the sulfidic component of the ore after Activators, flocculants, conditioning reagents, dispersing fine grinding, then the sulfides may be oxidized to liberate reagents, depressing reagents, etc. may also be used in the encapsulated gold and make it amenable to lixiviation. conjunction with the collectors employed in the present The oxidizing pretreatment is carried out prior to cyanida process. tion and may be any of the conventionally used oxidizing Dosages of collector ranging from about 0.1 to about 10 pretreatments for sulfide minerals. However, the selected lbs./ton of ore may be used, preferably at least about 0.5 pretreatment must be mild enough to avoid oxidation of the lb.fton. 65 carbonaceous component of the ore. Such oxidizing pre Contact of the slurry with the collector used in accordance treatments include autoclaving and bioleaching, especially with the present invention is preferably accomplished after with Thiobacillus ferrooxidans. Autoclaving is descried in 5,792.235 11 12 U.S. Pat. No. 4,610,724, hereby incorporated by reference. EXAMPLE 3 A bioleaching process that may be used in the present A sample of gold ore like the one in examples 1 and 2 was invention is described in Hutchins, et al., Microbial Pre pulverized the same way and then leached with 1000 ppm treatment of Refractory Sulfide and Carbonaceous Ores cyanide (1.73 g KCN/liter) in 0.1N NaOH at 40% solids for Improves the Economics of Gold Recovery, Mining 48 hours with open mixing at room temperature. The cya Engineering, April 1988, at 249, hereby incorporated by nide was removed by filtration and the wet ore resuspended reference. in 0.1N NaOH and 3% NaCL, the ore was then conditioned Alternatively, the sulfidic component of the ore may be at 10% pulp density with a collector of meadowfoam oil at separated from the carbonaceous ore by flotation and then 0.04 ml/liter. A concentrate was collected as before and treated separately using well known techniques such as 10 roasting to recover the occluded gold. analyzed for gold. The concentrate contained 78.3% of the The following examples are set forth for the purpose of total recovered gold in 16.7% of the weight. illustrating the invention only and are not to be construed as EXAMPLE 4 limitations on the present invention except as set forth in The same test was made on 48 hour leached ore using 5 appended claims. All parts and percentages are by weight 15 flotations and collections using 0.04 ml/liter Jojoba oil and unless otherwise specified. All of the carbonaceous ores used in the examples have the capacity of preg-robbingly Dowfroth 250. The concentrate contained 81.4% of the total removing about 140 pig Aufg ore in 16 hours or less from a recovered gold in 16% of the total weight of ore. cyanide solution spiked with 4 ppm Au. EXAMPLE 5 EXAMPLE 1. The same test was made on ore that was leached for 24 hrs. Five flotations were done with 0.04 ml/liter of mead A sample of carbonaceous gold ore from eastern Nevada owfoam oil and Dowfroth 250. The concentrate contained containing approximately 1% organic carbon and approxi 78% of the total gold in 17.8% of the total weight of ore. mately 0.15 oz.fton of gold was pulverized in a ball mill at about 60 to 70% solids. After 1.25 hrs. at 72 rpm the pulp 25 EXAMPLE 6 was diluted with water and passed through a 400 mesh sieve. The same test was made with an ore that was leached for The ore that did not pass the 400 mesh sieve was weighed 16 hr. The filtered ore was resuspended in 0.1N NaOH, 5% and found to be less than 5% of the total weight. The -400 (NH)SO and then floated 5 times with the meadowfoam mesh ore was made into a pulp of approximately 40% solids Dowfroth 250 method used in examples 2, 3, and 5. The with 1000 ppm CN (1.73 g KCN/liter) and 0.1NNaOH. The 30 concentrate contained 84.4% of the total recovered gold in final pH of the pulp was greater than 12.0. The carbonaceous 23.4% of the total ore weight. ore-cyanide pulp was mixed for 72 hours at room tempera ture. The solution was then removed by filtration and the wet EXAMPLE 7 ore resuspended in 0.05N NaOH and 3% by weight NaCl. A sample of carbonaceous gold ore containing 0.009 The cyanide treated ore was then conditioned at 10% pulp 35 oz?ton of gold and approximately 1% organic carbon was density in a Wemco 600 gmflotation cell with a collector of pulverized in a ball mill for 30 minutes to pass a 200 mesh jojoba oil at a concentration of 0.04 ml/liter. After 5 minutes sieve. The -200 mesh ore was leached with a 1000 ppm of mixing the same volume of Dowfroth 250 was added and cyanide and 0.1NNaOH solution for 16 hours. The cyanide mixed for an additional 5 min. Air was then introduced to was removed, and the ore was conditioned in 0.1N NaOH produce a blackcarbon containing froth which was collected and 5% by weight NaCl solution with a collector of mead for 5 min. The flotation process was repeated four more owfoam oil (0.04 ml/liter) for 5 minutes. Dowfroth 250 times with the addition of more collector and frother each (0.04 milliter) was added and the slurry was again condi time. Samples of the floated concentrate, the ore remaining tioned for 5 minutes. Air was introduced and black froth in the cell after flotation was complete, and the ground ore collected for over 20 minutes. The flotation process was at the start of the experiment were all analyzed for gold by 45 repeated 2 more times with about 10 minutes of collection the same method. The recovery of gold was calculated from each time. Samples of floated concentrates and tails were the weight of gold in the ore. The concentrate in example 1 dried and weighed and then analyzed for gold by the same had 76% of the total gold recovered in 20% of the total method. The concentrate contained 65% of the gold in 14% weight of ore. of the weight. The tail contained 1.25 ppm or 0.036 oz./ton 50 EXAMPLE 2 of gold or 34.5% of the gold in 86% of the weight. A sample of gold ore like the one used in example 1 was EXAMPLE 8 pulverized in a ball mill at about 60 to 70% solids for 1.25 A sample of the same ore used in example 7 was prepared hours at 72 rpm to pass a 400 mesh sieve. The ore passing in a similar manner except the ore was not leached with the 400 mesh sieve was leached with 1000 ppm cyanide 55 cyanide. The ore was floated with 3 additions of meadow (1.73 g KCN/liter) and in 0.1N NaOH at 40% solids. The foam oil (0.04 ml/l) and Dowfroth 250 (0.04 ml/l). Condi ore-cyanide pulp was mixed for 72 hours at room tempera tioning times were the same as in Example 7. The concen ture. The cyanide solution was removed by filtration and the trate contained 43% of the gold in 18% of the ore weight. wet ore was resuspended in 0.1N NaOH and 3% by weight The tail contained 2.0 ppm or 0.058 oz.fton of gold or 57% NaCl. The ore was then conditioned at 10% pulp density of the gold in 82% of the weight. with a collector of meadowfoam oil at 0.04 ml/liter. After 5 Therefore, a comparison of this example with Example 7 minutes of mixing the same volume of dowfroth 250 was illustrates the ability to the present invention to concentrate added and mixed. Then air was introduced and the froth gold using the preg-robbing component of the ore. collected for 5 minutes. This process was repeated four more times. Samples were analyzed for gold and the concentrate 65 EXAMPLE 9 contained 74.4% of the total gold in 14.4% of the total A sample of carbonaceous gold ore containing 0.085 weight. oz?ton gold and about 1% organic carbonaceous matter was 5,792,235 13 pulverized in a ball mill for 15 minutes. The ground ore was passed through a 400 mesh sieve. The +400 mesh ore was TABLE I ball mill ground again for 15 minutes and then passed CLTEST RESULTS OF WEMCOTALS. through a 400 mesh sieve. This process was repeated until at least 90% of the ore had passed the 400 mesh sieve. The %, Au From Au Au -400 mesh ore was leached with 1.000 ppm cyanide and Total pug Au Wenco Tail ppm oz./ton Gold in carbon 96.4 ug 29.6 3.85 0.12 0.1NNaOH for 16 hours. The cyanide was removed and the Gold in final 229 ug 70.4 0.82 0.024 ore conditioned in 0.1N NaOH, 5% NaCl, solution and tail floated with meadowfoam oil as a collector and Dowfroth 10 250 as a frother as in Example 7 and 8. Samples of floated A comparison of the results from Table I and II illustrates concentrates were dried, weighed and analyzed for gold. The that either a column flotation or CIL of the Wemco float tails concentrate contained 71% of the gold in 24% of the weight reduces the gold in the final tail to approximately 0.85 ppm and the tail contained 29% of the gold. A sample of tail was or about 0.025 oz/ton. Thus recovering about an additional also taken and tested in a laboratory column flotation cell of 15 25% of the gold in the Wemco tail. 40 cm.X5 cm. with a porous glass bottom into which air was Although the invention has been described with reference introduced to make small bubbles. After approximately 30 to preferred embodiments and specific examples, it will minutes the black concentrate was collected and analyzed readily be appreciated by those of ordinary skill in the art for gold. The column flotation was able to remove an that many modifications and adaptions of the invention are possible without departure from the spirit and scope of the additional 25% of the gold in the tail from the Wemco invention as claimed hereinafter. For example, while the flotation cell while only removing 5% of the gangue. processes according to the present invention have been described in terms of recovering gold from carbonaceous ores, the processes are equally applicable to other precious EXAMPLE 10 25 metals found in carbonaceous ores such as silver and plati l. We claim: A sample of carbonaceous gold ore containing 0.085 1. A process for recovering precious metal from refractory oz?ton gold was pulverized in a ball mill for 30 minutes. The ore, wherein said ore comprises indigenous adsorbing mate ground ore was left to settle out for 10 minutes, then all the 30 rial capable of adsorbing precious metal-lixiviant complexes ore that had not settled passed 36 cm. from the top of the and indigenous non-adsorbing precious metal containing original water level was removed. This method of settling material, said process comprising: was done to produce ore that was approximately 20 microns (a) contacting said refractory ore with a lixiviant to or smaller in size. This process was repeated three times and 35 thereby leach precious metal from said indigenous the +20 micron ore was reground for 30 minutes. After more non-adsorbing material and form precious metal than 90% of the ore was ground to less than 20 microns the lixiviant complexes; ore was leached in 1,000 ppm cyanide and 0.1N NaOH for (b) allowing said precious metal-lixiviant complexes to 16 hours. The cyanide was not removed but diluted to 500 adsorb on to said indigenous adsorbing material to form ppm and 0.1N NaOH with 5% NaSO added with mead a precious metal-enriched indigenous adsorbing mate owfoam oil (0.04 ml/l) for conditioning. The slurry was rial; conditioned for 5 minutes with meadowfoam oil (0.04 ml/l) (c) separating said precious metal-enriched indigenous as in the previous example. Flotation was done with Dow adsorbing material from said ore; and froth as a frother as before. A total of 3 flotations and (d) recovering precious metal from said precious metal 45 enriched indigenous adsorbing material. collections were done in a Wemco float cell before a sample 2. A process according to claim 1. wherein said method of from the tail was taken for column flotation. After the tail separating said precious metal-enriched indigenous material from the Wemco flotation was dried and weighed and a from said ore is froth flotation. sample removed for analysis, the remaining sample of 283 3. A process according to claim 1, wherein the precious grams was leached again in 1,000 ppm cyanide at 30% 50 metal recovered from said ore is at least one member density with 5% NaSO and 0.1NNaOH and 25gm/liter of selected from the group consisting of gold, platinum, and activated carbon. After 24 hours the carbon was removed by silver. filtration and the carbon and the remaining ore were ana 4. A process according to claim 1, further comprising the lyzed for gold. The results are listed below. step of augmenting said indigenous adsorbing material with 55 an adsorbing material capable of adsorbing precious metal TABLE lixiviant complexes. 5. A process according to claim 4, wherein said adsorbing RESULTS OF FLOTATION TESS material used to augment said indigenous adsorbing material Au Au is at least one material selected from the group consisting of 9, Gold % Weight ppm ozton activated carbon and recycled indigenous adsorbing mate Gold in Wemco 6783 26.4 6.72 0.195 rial. Concentrate 6. A process according to claim 1, wherein the lixiviant Gold in Wemco tail 30.10 68.4 1.15 0.033 used is a member selected from the group consisting of Gold in column .46 26 4.55 0.132 concentrate cyanide, aqua regia, and thiourea. 65 7. A process for recovering precious metal from refractory Gold in column tail 1.61 4.9 85 0.025 ore, wherein said ore comprises indigenous adsorbing mate rial capable of adsorbing precious metal-lixiviant complexes 5,792,235 15 16 and indigenous non-adsorbing precious metal containing 9. A process according to claim 7, further comprising material, said process comprising: hydrometallurgically treating the ore remaining after said (a) augmenting said indigenous adsorbing material with precious metal-enriched indigenous material is separated to an adsorbing material capable of adsorbing precious recover additional gold. 10. A process according to claim 7, wherein the lixiviant metal-lixiviant complexes; used is a member selected from the group consisting of (b) contacting said refractory ore with a lixiviant to cyanide, aqua regia, and thiourea. thereby leach precious metal from said indigenous 11. The process of claim 2 wherein the method of froth non-adsorbing material and form precious metal flotation is column flotation. lixiviant complexes; 12. The process of claim 1 wherein the method of (c) allowing said precious metal-lixiviant complexes to 10 separating the precious metal-enriched indigenous material adsorb on to said indigenous adsorbing material and from the ore is gravitational flotation. said adsorbing material added to augment said indig 13. The process of claim 1 wherein the precious metal enous adsorbing material to thereby form a precious removed from the ore is gold. metal-enriched indigenous adsorbing material and a 14. The process of claim 1 wherein the precious metal 15 removed from the ore is silver. precious metal-enriched adsorbing material; 15. The process of claim 1 wherein the precious metal (d) separating said precious metal-enriched indigenous removed from the ore is platinum. adsorbing material and said precious metal-enriched 16. The process of claim 1 wherein the lixiviant is adsorbing material from said ore; and cyanide. (e) recovering precious metal from said precious metal 17. The process of claim 1 wherein the lixiviant is aqua enriched indigenous adsorbing material and said pre regia, cious metal enriched adsorbing material. 18. The process of claim 1 wherein the lixiviant is 8. A process according to claim 7, wherein said method of thiourea, separating said precious metal-enriched indigenous material from said ore is froth flotation.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION

PATENT NO. : 5,792,235 DATED August 11, 1998 INVENTOR(S); William J. Kohr

It is certified that error appears in the above-identified patent and that said Letters Patent is hereby corrected as shown below: In Claim 16 (at column 16, line 12), delete "is gravitational flotation" and insert -comprises a technique selected from the group consisting of gravitational separation and froth flotation--.

Signed and Sealed this Eighth Day of February, 2000

Q. TODD DICKINSON Attesting Officer Commissioner of Patents and Trade narks